The present invention relates to machine tools, such as internal grinding machines, comprising a spindle device wherein a tool is attached to the forward end of a drivingly rotatable rotor spindle supported by a magnetic bearing device of the control type.
With internal grinding machines and like machine tools, the rotor spindle of the spindle device included therein is generally supported by usual bearings of the contact type such as antifriction bearings made of steel or antifriction bearings of ceramic. However, with a demand for improved productivities in recent years, there arises a need for high-speed rotation and higher feed speeds for working. Accordingly, it has been proposed to support the rotor spindle contactlessly by a magnetic bearing device of the control type.
Further for use in machine tools wherein the spindle device is contactlessly supported by such a control-type magnetic bearing device, a contact position detector has been proposed which is so adapted that when the exciting current of the magnetic bearing of the bearing device has exceeded a predetermined threshold value, this is interpreted as indicating the contact of the tool with the workplace (see Unexamined Japanese Patent Publication HEI 1-252343).
Machine tools such as internal grinding machines are generally so designed that the spindle device is moved at a high fast feed speed until the tool approaches the workpiece to some extent and thereafter moved at a low working feed-speed for working. If the mounted position of the tool or workplace involves an error, therefore, the tool is likely to collide with the workplace during fast feed. When the collision of the tool with the workpiece can be detected in such a case, damage to the tool is avoidable. Nevertheless, the conventional contact position detector merely checks whether the exciting current of the magnetic bearing has exceeded the threshold value to detect the contact of the tool with the workpiece and is accordingly unable to detect the degree of contact. The exciting current through the magnetic bearing exceeds the threshold value upon the tool colliding with the workpiece at the fast feed speed and while the tool in working on the workpiece at the working feed speed in contact therewith, so that the detector fails to distinguish between the abnormal collision and the usual working operation.